Transforming Activity of a Synthetic c‐Ha‐ras Gene Containing O 6 ‐Methylguanine in Codon 12

A mutagenic DNA‐adduct, O6‐methylguanine, was introduced into codon 12 of the synthetic c‐Ha‐ras gene by cassette mutagenesis. Transfection of this modified ras gene into normal NIH3T3 cells by the calcium phosphate procedure resulted in significant induction of focus formation. The ras gene inserted into the transformed cells was found to have a G to A transition at the position of the modified base. These results indicate that an O6‐methylguanine residue in DNA may lead to a mutation and be one cause of activation of the ras gene.